It is known in the art that the inclusion of Group V(B) compounds (e.g. tantalum carbide) to cemented carbide alloys used for cutting tools improves the wear resistance of the tools and also prevents tool failure due to galling and crater formation. Alloys containing up to 50% by weight of tantalum carbide dispersed throughout the alloy exhibit significantly better wear performance than alloys which do not contain tantalum carbide especially in the machining of depleted uranium alloys, high temperature alloys and stainless steels. However, because tantalum is a relatively expensive metal, alloys containing large amounts of tantalum are impractical and have not gained commercial acceptance.
Prior efforts to prevent galling and crater-type failure have focused on providing the alloys with at least one surface coating of, for example, titanium nitride, titanium carbide, aluminum oxide or combinations thereof. The surface coating is typically applied by chemical vapor deposition techniques known to those skilled in the art. Such coated hard metal alloys extend tool life in the machining of ferrous materials such as steels and cast irons. However, these coatings have not resulted in improved tools for the machining of uranium alloys and many high temperature alloys and thus the problems of galling and crater formation are still of concern to the industry.
It is also known in the art that hard metal alloys containing compounds of the Group IV(B) metals (e.g., titanium carbide) are advantageous because they have better cutting characteristics including superior crater and deformation resistance. Such alloys are also less expensive to produce and are lighter in weight. It would therefore be desirable and a significant advance in the industry to provide a hard metal alloy which incurs less galling and crater-type failure especially in the machining of those alloys described above which have had limited success using known coatings applied by chemical vapor deposition. It would also be desirable to reduce galling and crater-type failures in alloys which contain sufficient amounts of Group IV(B) compounds to thereby obtain the benefit of the superior cutting characteristics associated with such compounds.
The present invention was arrived at by considering that galling and crater formation occur at or near the surface of the tool during machining. Since Group V(B) compounds (e.g., tantalum carbide) are known to inhibit such deleterious formations, it was deemed desirable to form a surface region enriched with these compounds in alloys which contain Group IV(B) compounds and those that do not. In the course of this investigation, applicant discovered that the Group IV(B) compounds, especially titanium carbide, inhibit the migration of the Group V(B) compounds to the surface of the hard metal alloy. This discovery led to the present invention wherein a hard metal alloy contains at least one Group V(B) compound and optionally at least one Group IV(B) compound wherein the ratio of the atomic percent of the Group IV(B) compounds to the atomic percent of the Group V(B) compounds is from 0 to about 1.5. Within this critical range, there is sufficient migration of the Group V(B) compounds to the surface of the alloy to obtain the desired objects of the present invention.
Previous work has been performed on alloy compositions containing both Group V(B) and Group IV(B) compounds. Typically these alloys have been treated to form surface regions enriched with binder metals (e.g., cobalt and tungsten carbide). However, prior to the present invention, no one has recognized the criticality of employing alloy compositions containing Group V(B) and Group IV(B) compounds in the above-mentioned range and thereby obtain a surface region enriched with Group V(B) compounds especially in alloy compositions containing Group IV(B) compounds.
For example, U.S. Pat. No. 3,999,954 (Kolaska et al.) describes the production of a wearresistant hard metal alloy containing tungsten carbide, tantalum carbide, titanium carbide and a binder metal such as cobalt. A surface coating of titanium carbide is applied by customary chemical vapor deposition techniques. Thereafter the alloy is heated under conditions of temperature and pressure sufficient to cause the cobalt to migrate to the surface. Intermediate coating layers containing binder metal are also described.
U.S. Pat. No. 4,018,631 (Hale) describes a process for forming an oxidized coating on a cemented carbide alloy containing tungsten carbide, tantalum carbide, titanium carbide and cobalt.
U.S. Pat. No. 4,282,289 (Kullander et al.) discloses a process for the production of cemented carbide products by treating the substrate to form a carbide, nitride or carbonitride coating and during or after such treatment, adding a sulfide or nitride gas to form sulfide and/or nitride portions on or in the coating.
Kullander et al., U.S. Pat. No. 4,399,168 discloses a method of heat treating a coated substrate at a temperature of at least about the melting point of the binder phase to diffuse the binder from the substrate into the coating and then applying a second coating which in turn is treated with an oxygen-, nitrogen- or sulfur-containing gas.
All of the aforementioned U.S. Patent citations are incorporated herein by reference.
The prior art methods mentioned above utilize alloys containing Group V(B) compounds (e.g., tantalum carbide) and Group IV(B) compounds (e.g., titanium carbide) and subject the alloys to temperature and pressures which enable a binder metal such as cobalt and tungsten carbide to diffuse to the surface. None of the references teach a Group V(B) compound enriched surface region (e.g., a surface region enriched with tantalum carbide) nor is there any teaching of the criticality of employing applicant's ratio of the atomic percent of Group IV(B) compounds to the atomic percent of Group V(B) compounds in the range of from 0 to about 1.5 to thereby obtain a surface region enriched with the Group V(B) compounds.
Re. Pat. No. 28,485 (Rix et al.) exemplifies a hard metal body containing a tantalum compound in the absence of a Group IV(B) compound. However, the method described in this patent requires heating the alloy to temperatures (e.g., 1000.degree. C. ) well below the temperature needed to diffuse tantalum carbide to the surface of the alloy. Indeed, the patent specifically teaches that the surface area is not formed from a solid solution phase and thus tantalum carbide can not migrate to the surface under the Rix et al. process parameters.
It is therefore an object of the present invention to provide a method of producing a cemented carbide hard metal alloy with or without the presence of Group IV(B) compounds having a surface region enriched with the carbides, nitrides and carbonitrides of tantalum, niobium and vanadium or combinations thereof.
It is a further object of the invention to produce a cemented carbide hard metal alloy which exhibits superior resistance to galling and crater formation when used as a cutting tool to machine uranium alloys, high temperature alloys and ferrous materials.